Diesel fuel is a refined petroleum product which is burned in the engines powering most of the world's trains, ships, and large trucks. Petroleum is, of course, a non-renewable resource of finite supply. Acute shortages and dramatic price increases in petroleum and the refined products derived from petroleum have been suffered by industrialized countries during the past quarter-century. Furthermore, diesel engines which run on petroleum based diesel emit relatively high levels of certain pollutants, especially particulates. Accordingly, extensive research effort is now being directed toward replacing some or all petroleum-based diesel fuel with a cleaner-burning fuel derived from a renewable source such as farm crops.
In an effort to partially replace dependence on petroleum based diesel, vegetable oils have been directly added to diesel fuel. These vegetable oils are composed mainly of triglycerides, and often contain small amounts (typically between 1 and 10% by weight) of free fatty acids. Some vegetable oils may also contain small amounts (typically less than a few percent by weight) of mono- and di-glycerides.
Triglycerides are esters of glycerol, CH2(OH)CH(OH)CH2(OH), and three fatty acids. Fatty acids are, in turn, aliphatic compounds containing 4 to 24 carbon atoms and having a terminal carboxyl group. Diglycerides are esters of glycerol and two fatty acids, and monoglycerides are esters of glycerol and one fatty acid. Naturally occurring fatty acids, with only minor exceptions, have an even number of carbon atoms and, if any unsaturation is present, the first double bond is generally located between the ninth and tenth carbon atoms. The characteristics of the triglyceride are influenced by the nature of their fatty acid residues.
The production of alkyl esters from glycerides can occur by transesterification. However, transesterification suffers in that the reaction generally requires the addition of an acid or base catalyst which must be first neutralized thereby generating salts and soaps. In addition, while transesterification results in the separation of fatty acid esters from triglycerides, it also results in the production of glycerin, which must then be separated from the fatty acid esters, glycerin, excess alcohol, salts, and soaps. Furthermore, the use of a strong acid, such as sulfuric acid, typically leads to higher sulfur content in the resulting biodiesel as the acid reacts with the double bonds in the fatty acid chains.
In an effort to overcome some of the problems associated with the production of carboxylic acid esters and biodiesel, the present invention employs reactive distillation as a method to assist in the production of biodiesel fuel having low glycerin, water, and sulfur content. Reactive distillation is also useful in producing fatty acid esters from feedstock containing relatively high concentrations of glycerides in the feed to the esterification step. Reactive distillation is a method wherein specific reactions which are affected by an unfavorable equilibrium position of the main reaction, wherein during the reaction one or more substances are continuously removed from the reaction mixture. Sulfur content is reduced by employing reactive distillation over a solid catalyst bed and free glycerin concentration is reduced by employing fat hydrolysis.